Ion Acoustic Oscillations in Plasma Thermionic Energy Converters

Abstract

The oscillations in low‐pressure plasma thermionic energy converters are ion acoustic oscillations that may be explained on the basis of an electron beam‐plasma interaction. A low‐energy beam is formed by the acceleration of electrons through an ion sheath at the emitting surface. The ion acoustic oscillations are observed as expected when the ordered beam velocity is greater than the phase velocity of the wave and less than the plasma electron thermal velocity. Their frequency is ${\mathrm{f=\lambda }}^{\text{−1}}{\mathrm{(\gamma }}_e{\mathrm{kT}}_e{\mathrm{/m}}_i{)}^{\frac{1}{2}}$ where the emitter to collector spacing is a multiple of the half‐wavelength. Langmuir probes indicate high effective electron temperatures, and the agreement between calculated and measured frequencies is good if γ_e=3 is used, whether or not the electron‐electron collision frequency is higher than the frequency of the observed oscillations.